Introduction to CH-acidic Compounds

β-Dicarbonyl Compounds

The acidity of β-dicarbonyl compounds is considerably higher than that of monocarbonyl compounds and other dicarbonyl compounds. In keto-enol equilibrium of β-dicarbonyl compounds, the (mono)enolic form usually exceeds the keto form. 2,4-Pentadione, for instance, consists of about 76 percent enolic form.

Fig.1

β-Dicarbonyl compounds.

The reason which may be provided for the (mono)enolic form's excess in the keto-enol equilibrium of β-dicarbonyl compounds is the noticeably higher stabilization of the enol in comparison to other carbonyl compounds. While, on the one hand, a β-dicarbonyl compound's enol is additionally stabilized through resonance of the enol's carbon-carbon double bond with the second carbonyl group, the enol's hydroxy hydrogen is, on the other hand, connected to the carbonyl oxygen by a hydrogen bridge bond. The formation of this hydrogen bridge bond is promoted by the planar structure of the enol-carbonyl resonance system because this leads to an ideal spatial orientation of the hydroxy group and carbonyl group in order to construct a strong hydrogen bridge bond. All in all, the monoenolic form of a β-dicarbonyl compound has a planar, six-membered cyclic structure, which is stabilized by resonance and a hydrogen bridge bond.